A map of key-value-pairs. The values are hold in a list, so adding the same
key-value-pair twice to the map, will result in storing the value twice.
Even the sequence of adding the values will be saved.

The name AccuMap is from accumulation map. You can use this map to easily
storing all you key-value-pairs. After that you can all values-lists by
key. Unlike the MultiMap, you don't lose the information of identical values
and their order of adding.